1. Field of the Invention
This invention relates to image display apparatus for displaying images on the screen of a CRT (cathode ray tube) or other display device.
2. Description of the Related Art
In a typical electronic image display apparatus and method the pixel information for an image to be displayed is composed of discrete pixels each of which is represented by a respective digital code which defines a pixel in terms of luminance and chrominance pixel component values. Each of these digital codes is stored in a display memory which is addressed periodically in synchronism with the operation of the display device by a display generator which reads out the digital codes and produces therefrom video signals for driving the display device. The apparatus can include or have associated with it a background memory in which is stored the pixel information for a large number of images. When a displayed image is to be replaced by a new image, the pixel information for the new image is made available in the display memory as the respective digital codes which represent the pixel component values for the new image. The pixel information can be stored in the background memory as the actual respective digital codes which represent the pixel component values, so that the digital codes for the new image can be read out from the background memory and written directly into the display memory in place of the digital codes for the previously displayed image.
The time taken for this read/write operation in respect of the digital codes for the new image depends inter alia upon the operating speed (i.e. the speed of data access) of the background memory. When the background memory is a mass memory, such as an optical record carrier (i.e. a compact disc) in its role as a read only memory (CD ROM), its operating speed may be too low for certain applications. In particular, it has been determined empirically that a user of image display apparatus of the above type will tolerate a delay of only about one second for replacing a displayed image by a new image. If the delay is significantly longer, then use of the apparatus becomes aesthetically unacceptable. As a consequence, the amount of pixel information that can be transferred from the background memory to the display memory to change the image being displayed becomes limited.
It has been found that such a limitation poses a problem in respect of enhanced resolution displays which require a large amount of pixel information. A CD ROM can readily provide the storage capacity which is necessary, but accessing this pixel information sufficiently quickly has proven to be difficult. As an example, for a normal resolution image display using, say, a 360.times.280 pixel matrix, the time taken to load digital codes representing pixel component values from a CD ROM into a display memory is thought to be on the margin of user acceptability. However, for an enhanced resolution image display using, say, a 760.times.560 pixel matrix, (i.e. having a quadruple number of pixels), which is contemplated, the loading of digital codes representing pixel component values will take four times longer, which is unacceptable.
In order to reduce the amount of digital data that is required to be stored to provide enhanced resolution image displays, there is proposed in Applicants' prior British Patent Application No. 8609078 corresponding to U.S. Application Ser. No. 035,104, filed Apr. 6, 1987, a method of image coding by which pixel information for at least one pixel component value of a plurality of pixels forming an image is coded into two sets of digital data of which, one set represents values for the difference between pixel component values for an image of a given (enhanced) resolution and pixel component values for an image of lower resolution of the same pixel density, and the other set represents a lesser number of pixel component values for the lower resolution image but of reduced pixel density. A complementary decoding method expands by interpolation the lesser number of pixel component values to that of the enhanced resolution image and the latter is then restituted by combining this expanded number of pixel component values with the difference values.
The coding of the difference values into a set of digital data preferably consists in providing these values initially as pulse code modulated (PCM) codes and then quantizing them into a smaller number of quantizing values, including zero, and coding the quantized values using a run length code. Significant data compression is achieved by this coding, particularly as there will nearly always be a large number of zero difference values. Still further data compression can be realised by providing the pixel component values initially as PCM codes and then delta coding them to form the other set of digital data as delta pulse code modulated (DPCM) codes.
In the image display apparatus described in the aforementioned patent application, the digital codes stored in the display memory and representing the pixel information for an image to be displayed are DPCM codes. As a consequence, this apparatus requires a combined decode/encode device at its input, initially to decode the two sets of digital data read out from the background memory in respect of an image to be displayed, into PCM codes. The PCM codes which are formed from the set of digital data which represents the pixel component values for a lower resolution image can be coded directly into DPCM codes for storage in the display memory when only the lower resolution image is to be displayed. For the display of the enhanced resolution image, these PCM codes are expanded in number by interpolation and then combined with the PCM codes which are formed from the other set of digital data which represents the difference values. The resultant PCM codes which represent the pixel component values for the enhanced resolution image are then coded into DPCM codes for storage in the display memory.
However, this use of the display memory by means of DPCM codes has the limitation that for an enhanced resolution image display, it requires much more display memory than is required for the display of the corresponding lower resolution image. In fact, in the example given in the aforementioned patent application, the amount of display memory which is needed is increased by a factor of four. Also, the necessity of including a DPCM encoder at the input of the display memory increases the complexity of the apparatus and can introduce further quantization errors.